1. Field of the Invention
The invention generally relates to a method of fabricating a solar cell with low reflectivity.
2. Description of Related Art
Solar cell is a very promising clean energy source, which can generate electricity directly from sunlight. However, the efficiency of solar cells needs to be further enhanced, so that solar cells can be widely accepted as a major electricity source. In general, the efficiency of solar cells is highly relevant to the light absorption mechanism and the usage of light generating carriers.
Recently, all high-efficiency single crystalline silicon solar cells have a particular textured structure on the front surface, so as to reduce the reflection losses and to improve the light-trapping property. Currently, the most effective texturization method is to etch pyramids on monocrystalline wafers with (100) surface. This method is usually performed by using alkaline etching solutions of potassium hydroxide (KOH) or sodium hydroxide (NaOH) with anisotropic etching rates, where the etching rate of (111) surface is slower than that on (100) and (110) surfaces by two orders of magnitude. Therefore, randomly distributed pyramids are formed through intersecting (111) surface, which are called random pyramids, or inverted pyramids are formed by using a cross-hatched masking layer. However, as the multiple grains of multicrystalline wafers have different orientations, the method aforementioned is not suitable for multicrystalline wafers.
On the other hand, when the conventional alkaline etching technique is used, the random grain orientations of the multicrystalline silicon wafer have an inhibited effect in forming the uniform and effective surface configuration. Therefore, the isotropic etching or other approaches that are not affected by the crystal orientation must be adopted to texturize the surface of multicrystalline silicon wafer.
As for a wet chemical isotropic etching of multicrystalline silicon, hydrofluoric acid (HF) and nitric acid (HNO3) are more suitable. The reaction of these acids with silicon is strongly exothermic. Generally, the silicon substrate processed with this traditional acid etching treatment usually has higher reflectivity.
In addition, a wet chemical approach for texturing multicrystalline silicon has been disclosed in U.S. Pat. No. 6,790,785, where a nanometer scale porous layer is etched on a multicrystalline silicon surface. The weighted reflectance values obtained by this approach is reduced to 9%, but the short-circuit current density of the solar cell is significantly reduced. That's because the porous silicon etching may result in poor surface recombination loss and high absorption coefficient for short wavelength photons. Furthermore, as the direct bandgap behavior of the short wavelength response is reduced, the porous silicon layer has an appreciable absorption loss.
Furthermore, in some research, the silicon surface is textured by first disposing a metal-containing layer, for example, as disclosed in WO 2007/025536, and the etching process is performed with a solution containing hydrofluoric acid (HF) or ammonium fluoride (NH4F), as well as an oxidizing agent. However, it is quite complicated to control the thickness of the metal-containing layer in terms of a large surface area.
Thus, a method for the silicon substrate to have a surface structure with low reflectivity is needed to improve the light absorption efficiency of the solar cell and enhance the performance of the solar cell.